Wizardry: Proving Grounds of the Mad Overlord

The Mad Overlord Trebor was once only power-mad, but went off the deep end after he acquired a magical amulet of immense power, only to have it stolen from him by his nemesis, the evil archmage Werdna.

Werdna, not quite sure how to use the amulet properly, accidentally causes an earthquake which creates a ten-level dungeon beneath Trebor's castle. To avoid looking silly, Werdna declares the dungeon to be the new lair for him and his monster hordes.